Kerma stands for “kinetic energy released in matter” (sometimes styled “per unit mass”). It measures the total kinetic energy that uncharged radiation, such as X-rays, gamma rays, or neutrons, transfers to charged particles at a specific point in a material. The SI unit for kerma is the gray (Gy), equal to one joule of energy per kilogram of material. If you’ve encountered kerma on a radiology report or in a physics course, it is closely related to absorbed dose but describes a different step in the chain of energy transfer.
How Kerma Works
When a photon (an X-ray or gamma ray) enters tissue, it doesn’t deposit its energy directly. Instead, it knocks electrons loose from atoms. Those freed electrons then travel through the surrounding material, ionizing and exciting other atoms along the way. Kerma captures the first part of that two-step process: it quantifies how much kinetic energy the incoming radiation hands off to the electrons at the point of interaction. It does not track what those electrons do afterward or where they eventually deposit their energy.
Because kerma describes energy transferred rather than energy absorbed, it is only defined for indirectly ionizing radiation. That means photons and neutrons. It is not used for charged particles like alpha or beta particles, which ionize matter directly.
Kerma vs. Absorbed Dose
Kerma and absorbed dose share the same unit (the gray) and are easy to confuse, but they answer different questions. Kerma asks: how much energy did the radiation release to charged particles at this point? Absorbed dose asks: how much energy was actually deposited in the material at this point? The Nuclear Regulatory Commission summarizes the distinction simply: kerma measures energy released by the radiation, while absorbed dose measures energy absorbed by the material.
These two events do not happen in the same place. The transfer of energy (kerma) occurs upstream from where the dose is ultimately deposited, because the freed electrons travel some distance before giving up all their energy. When a photon beam enters a material, kerma starts immediately at the surface, but absorbed dose builds up gradually as electrons from upstream interactions arrive. Dose reaches a maximum at a depth roughly equal to the range of those secondary electrons. Beyond that depth, kerma and absorbed dose track very closely together.
This buildup effect has a practical consequence you can actually feel. High-energy photon beams from cobalt-60 or medical linear accelerators spare the skin because kerma at the surface is relatively high but absorbed dose there is still low. The electrons set in motion at the surface carry their energy deeper before depositing it. This “skin-sparing” property is one of the reasons megavoltage beams are preferred for treating deep tumors.
Collision Kerma and Radiative Kerma
Once electrons are freed, they lose their kinetic energy in two ways. Some energy goes into ionizing and exciting nearby atoms, producing the biological or chemical effects we care about. The rest is re-emitted as new photons, primarily through a process called bremsstrahlung, where an electron decelerates near an atomic nucleus and radiates energy away.
Total kerma is split accordingly into two components. Collision kerma accounts for the portion of electron energy spent on ionization and excitation. Radiative kerma accounts for the portion converted back into photon energy. In most diagnostic and therapeutic photon beams, the radiative fraction is small (less than 1% for cobalt-60 gamma rays), so collision kerma and total kerma are nearly identical. At very high photon energies, the radiative fraction grows larger and the distinction becomes more important.
When Kerma Equals Absorbed Dose
Under a condition called charged particle equilibrium, the collision kerma at a point equals the absorbed dose at that point. This equilibrium exists when, for every electron carrying energy out of a small volume, another electron carries the same amount of energy in. In practice, this condition is met at depths beyond the dose buildup region in a uniform medium, as long as you are far enough from the beam edges.
For cobalt-60 beams, the factor connecting kerma and dose in the equilibrium region is so close to 1.0 (about 0.996 multiplied by a small correction of roughly 1.005) that the two quantities are effectively equal. Even at much higher photon energies used in modern radiotherapy, the product of these correction factors stays close to unity, so kerma remains a reliable stand-in for dose at sufficient depth.
How Kerma Is Calculated
For a beam of photons with a single energy, kerma at a point in a given material equals the photon energy fluence at that point multiplied by a property of the material called the mass energy-transfer coefficient. Energy fluence is simply the number of photons crossing a unit area multiplied by their individual energy. The mass energy-transfer coefficient describes how efficiently the material converts photon energy into electron kinetic energy, and it varies with both photon energy and the type of material (air, water, bone, soft tissue).
This material dependence means that the same photon beam produces different kerma values in different substances. A beam passing through air generates “air kerma,” while the same beam in soft tissue generates “tissue kerma.” The two can differ noticeably because tissue contains heavier elements than air and interacts with photons differently, especially at lower energies where photoelectric absorption dominates.
Air Kerma in Medical Imaging
In diagnostic radiology and fluoroscopy, air kerma is the standard way to quantify radiation output. It is measured in air, free from the complicating effects of backscatter from a patient’s body. For diagnostic X-ray energies, air kerma is essentially equal to the dose delivered to that small volume of air.
Two specific air kerma quantities appear frequently on fluoroscopy equipment. Reference air kerma is the accumulated air kerma at a defined reference point near the patient’s skin. For C-arm fluoroscopes, that reference point sits along the central X-ray beam, 15 cm back from the system’s center of rotation toward the X-ray tube. It serves as an estimate of how much radiation the patient’s skin received during a procedure.
Kerma-area product (KAP), also called dose-area product, takes air kerma one step further by integrating it across the entire cross-section of the X-ray beam. KAP acts as a surrogate for the total energy delivered to the patient and is the quantity recommended by the International Commission on Radiation Units and Measurements for monitoring patient dose during interventional radiology. Most modern fluoroscopes display both reference air kerma and KAP in real time, giving the operator continuous feedback on cumulative patient exposure.
Why the Distinction Matters
Kerma exists as a separate quantity because it is often easier to measure or calculate than absorbed dose, especially in a beam of photons moving through a complex geometry. At diagnostic energies, where the range of secondary electrons is tiny (fractions of a millimeter), kerma and dose are practically the same everywhere. At therapeutic energies, where electrons can travel centimeters, understanding the spatial offset between kerma and dose is essential for accurate treatment planning. Knowing where energy is released versus where it is deposited is the difference between predicting skin reactions correctly and missing them entirely.